One non-destructive tomographic measurement technology used in the medical field, etc., is the optical coherence tomography (OCT) that uses light of temporally low coherence as a probe (refer to Patent Literature 1). As it uses light as a measurement probe, the OCT has the advantage of being able to measure the refractive index profile, spectral information, polarization information (birefringence index profile), etc. of the object to be measured.
A basic OCT 43 is based on Michelson's interferometer whose principles are explained using FIG. 3. Light emitted from a light source 44 is paralleled by a collimator lens 45 and then split into reference light and object light by a beam splitter 46. The object light is focused onto an object to be measured 48 by an objective lens 47 in the object arm, where it is scattered/reflected and then returned to the objective lens 47 and beam splitter 46.
On the other hand, the reference light passes through an objective lens 49 in the reference arm and then gets reflected by a reference mirror 50 and returns to the beam splitter 46 through the objective lens 49. The object light and reference light that have thus returned to the beam splitter 46 enter a condensing lens 51 and get focused onto a light detector 52 (photo-diode, etc.).
For the light source 44 of the OCT, a source of light of temporally low coherence (lights emitted from the light source at different times are extremely unlikely to interfere with each other) is used. With Michelson's interferometer that uses temporally low coherence light as its light source, interference signals manifest only when the distance from the reference arm is roughly equal to the distance from the sample arm. Accordingly, one can measure the intensities of interference signals using the light detector 52 while varying the optical path length difference (τ) between the reference arm and sample arm to obtain interference signals at different optical path length differences (interferogram).
The shape of this interferogram represents the reflectance distribution of the object to be measured 48 in the depth direction, so the structure of the object to be measured 48 in the depth direction can be obtained by one-dimensional scan in the axial direction. With the OCT 43, therefore, the structure of the object to be measured 48 in the depth direction can be measured by scanning the optical path length.
A two-dimensional section image of the object to be measured can be obtained by a two-dimensional scan combining the aforementioned scan in the axial direction with a mechanical scan in the lateral direction. A scanner that performs this scan in the lateral direction may be configured, for example, to directly move the object to be measured, to shift the objective lens while keeping the object fixed, or to rotate the angle of the galvano-mirror placed near the pupil surface of the objective lens while keeping both the object to be measured and objective lens fixed.
An advanced version of the basic OCT mentioned above is the spectral domain OCT (SD-OCT) that obtains spectral signals using a spectrometer, and the swept source OCT (SS-OCT) that scans the wavelengths of the light source to obtain spectral interference signals. The SD-OCT has two types: the Fourier domain OCT (FD-OCT) (refer to Patent Literature 2) and the polarization-sensitive OCT (PS-OCT) (refer to Patent Literature 3).
The FD-OCT is characterized by obtaining the wavelength spectrum of reflected light from the object to be measured using the spectrometer and applying Fourier conversion to the intensity distribution of this spectrum to extract signals in the actual space (OCT signal space), thereby measuring the section structure of the object to be measured simply by scanning it in the x-axis direction, without having to scan it in the depth direction.
The SS-OCT obtains a three-dimensional optical tomographic image by rearranging and processing the interference signals using the light-source scanning signals obtained synchronously with the spectral signals by changing the wavelength of the light source with a high-speed swept source laser. The SS-OCT can also use a monochrometer as a means for changing the wavelength of the light source.
Retinal blood flow distribution measurement using the Doppler optical coherence tomography (Doppler OCT) is known. The Doppler-OCT provides a means for measuring the blood flow distribution in the retina using the aforementioned FD-OCT, etc., and by combining the Doppler-OCT with the spectral domain OCT, it becomes possible to form cross-section retinal blood flow images and also observe the dimensional vascular structure of the retina.
The inventors of the present invention had focused on the Doppler-OCT and studied and developed non-invasive ways to measure blood flows in a living body, especially those at the fundus of the eye. The inventors of the present invention had succeeded in inspecting the blood flows at the fundus of the eye by using the SD-OCT as the technical basis and installing the Doppler-OCT on it. However, the technical basis of OCT has been shifting from the SD-OCT to the next-generation technology SS-OCT in recent years.